1. Field of the Invention
The present invention relates to a method for grouping relay stations in a wireless multi-hop relay communication system and a system thereof. More particularly, the present invention relates to a scheduling method for a wireless multi-hop relay communication system for improving the transmission efficiency and capacity of the system.
2. Description of Related Art
Next generation mobile communication systems are envisioned to provide high-speed, high link quality, and high security transmissions, and are also expected to support various communication services. An effective resource schedule/allocation method has to be established to meet different quality of service (QoS) requirements from different users. Users located at cell boundary have worse link quality due to the long transmission distance to the base station, and users in the cell with severe shadowing effect also have worse link quality, thereby the foregoing users cannot perform high-speed data transmissions. To resolve the foregoing problem, the deployment density of base stations can be increased to shorten the propagation distances between the base stations and users so as to improve the link quality; or more base stations can be deployed at those areas with severe shadowing for improving the link quality of users in the areas. However, the cost of the base stations and the cost of the backhaul network connections will be substantially increased by the aforementioned method. On the other hand, the transmission power of the base station can be increased to improve the link quality and to reduce the cost of the base station. However, if the transmission power is increased, not only the transmission cost will be increased but also the interference level will be increased.
Multi-hop relay cell architecture is a good solution when considering all factors such as QoS, deployment cost, transmission power, and coverage area of the cell. Relay stations can be deployed within a cell to relay information from a base station to mobile stations with worse link quality, and vise versa. It has been shown that using relay stations may improve cell coverage, user throughput and system capacity.
Relay stations may be deployed at areas with severe shadowing or near the cell boundary, the users who can not be directly served by base station may be served by the relay stations, therefore the effective coverage area of the base station can be extended.
A single link with worse quality is divided into a plurality of links with better quality so that each of the links can provide higher transmission rate. However, since the same data will be duplicated and relayed over the air multiple times for multi-hop transmissions, it consumes the radio resources.
Besides, since there are a base station and several relay stations in a cell, to improve the spectrum efficiency, multiple serving stations may be active simultaneously if the potential interference is tolerant.
To obtain benefits for multi-hop relay communication systems, an efficient scheduling mechanism is required to arrange the transmissions of base stations and relay stations.
To improve the performance of a wireless communication system, a method of relay stations deployment in a Manhattan-like environment was provided in the Wireless World Initiative New Radio (WINNER) program. The Manhattan-like environment is a grid environment wherein the width of blocks is about 200 m and the width of streets is about 30 m.
FIG. 2 illustrates a first layout of relay stations in a Manhattan-like environment, wherein a base station 205 and four relay stations 201˜204 are disposed, and the base station and the relay stations all communicate with users through omni-directional antennas. However, since the relay stations are disposed outside of the coverage area 206 of the base station, each relay station requires an additional directional antenna pointing at the base station for communicating with the base station, and which increases the hardware cost of the relay stations.
FIG. 3 illustrates the transmission scheduling in such structure, wherein frame structures are transmitted within a single cell. The frame S301 may be divided into two sub-frames S302˜S303. The first sub-frame S302 is further divided into 5 time slots S304˜S308, wherein the base station 305 serves the 4 relay stations 301˜304 during the first 4 time slots S304˜S307 respectively and the base station 305 serves users within area 306 which is directly connected to the base station during the fifth time slot S308. The second sub frame S303 is divided into two time slots S309˜S310, and with the characteristics of spatial separation of the environment, the relay stations 301 and 302 serve users within the areas 307 and 308 connected thereto during the same time slot S309, and the relay stations 303 and 304 serve users within the areas 309 and 310 connected thereto during another time slot S310.
FIG. 4 illustrates the layout of relay stations in a multi-cell structure, wherein the coverage area 406 of a single cell A and the coverage area 416 of a single cell B are arranged in a staggered way. The base stations 405 and 415 in FIG. 4 respectively represent the positions of the base stations in cell A and cell B. The relay stations 401, 402, 403, and 404 belong to cell A, and the relay stations 411, 412, 413, and 414 belong to cell B. The arrangement of transmission frames thereof is shown in FIG. 5, wherein the arrangement of transmission frames between adjacent cells is to permute the operation orders of the sub-frames S502˜S503 in a frame S501 so that interference between cells can be prevented. The main purpose of the relay stations is to extend the coverage area of the base station, however, the link quality of users at the boundary of the service range of the base station cannot be improved. Besides, all the base station are idled for some time durations in the frame structure, since base stations are the only serving stations connected to the backhaul networks and carrying the effective data, the transmission efficiency of the base station in this design is not ideal.
FIG. 6 illustrates the second layout of a base station 605 and four relay stations 601˜604 in a Manhattan-like environment, wherein the base station and the relay stations all communicate with users by using omni-directional antennas. Since the relay stations 601˜604 are disposed within the coverage area 606 of the base station, no additional directional antenna is required by each relay station for communicating with the base station and in the design, the link quality of users in the cell boundary can be improved.
In this layout with all serving stations equipped with omni-directional antennas, the feasible transmission scheduling is shown as FIG. 7. FIG. 7 illustrates the transmission frame structure in a single cell, wherein the base station 705 respectively serves the four relay stations 701˜704 sequentially during the first four time slots S701˜S704, and at the same time, the base station 705 serves users directly connected to the base station 705. The relay stations 701 and 703 serve users connected thereto during the time slot S705. After that, the relay stations 702 and 704 serve users during the next time slot S706. The main purpose of such a layout is to improve the link quality of users at cell boundary; however, a complete transmission within a single cell requires at least 6 phases to be completed. When considering the multi-cell structure, because of the use of omni-directional antennas, the reuse factor of at least 2 is required to avoid the severe inter-cell interference, and thus decreases the overall system capacity.
Regardless of the first layout or the second layout that all serving stations are equipped with omni-directional antennas, all the base station and the relay stations are idled for some time in the frame structure, thus, the transmission efficiency thereof is not ideal.